Method of Fabricating A Semiconductor Device

ABSTRACT

A semiconductor device with high reliability is provided using an SOI substrate. When the SOI substrate is fabricated by using a technique typified by SIMOX, ELTRAN, or Smart-Cut, a single crystal semiconductor substrate having a main surface (crystal face) of a {110} plane is used. In such an SOI substrate, adhesion between a buried insulating layer as an under layer and a single crystal silicon layer is high, and it becomes possible to realize a semiconductor device with high reliability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device fabricated byusing an SOI (Silicon on Insulator) substrate and a method offabricating the same. Specifically, the invention relates to asemiconductor device including a thin film transistor (hereinafterreferred to as TFT) formed on an SOI substrate.

Incidentally, in the present specification, the semiconductor deviceindicates any device capable of functioning by using semiconductorcharacteristics. Thus, the semiconductor device includes not only a TFTbut also an electro-optical device typified by a liquid crystal displaydevice or a photoelectric conversion device, a semiconductor circuit inwhich TFTs are integrated, and an electronic equipment containing suchan electro-optical device or a semiconductor circuit as a part.

2. Description of the Related Art

In recent years, VLSI techniques have been remarkably developed, andattention has been paid to an SOI (Silicon on Insulator) structure forrealizing low power consumption. This technique is such a technique thatan active region (channel formation region) of an FET, which has beenconventionally formed of bulk single crystal silicon, is made thin filmsingle crystal silicon.

In an SOI substrate, a buried oxide film made of silicon oxide exists onsingle crystal silicon, and a single crystal silicon thin film is formedthereon. Various methods of fabricating such SOI substrates are known.As a typical SOI substrate, an SIMOX substrate is known. The term SIMOXis an abbreviation for Separation-by-Implanted Oxygen, and oxygen is ionimplanted into a single crystal silicon substrate to form a buried oxidelayer. The details of the SIMOX substrate are disclosed in [K. Izumi, M.Docken and H. Ariyoshi: “C.M.O.S. devices fabrication on buried SiO₂layers formed by oxygen implantation into silicon”, Electron. Lett., 14,593-594 (1978)].

Recently, attention has also been paid to a bonded SOI substrate. Thebonded SOI substrate realizes the SOI structure by bonding two siliconsubstrates as suggested by its name. If this technique is used, a singlecrystal silicon thin film can be formed also on a ceramic substrate orthe like.

Among the bonded SOI substrates, in recent years, attention has beenespecially paid to a technique called ELTRAN (registered trademark byCanon K.K.). This technique is a method of fabricating an SOI substrateusing selective etching of a porous silicon layer. The particulartechnique of the ELTRAN method is disclosed in, K. Sakaguchi et al.,“Current Progress in Epitaxial Layer Transfer (ELTRAN)”, IEICE TRANS.ELECTRON. Vol. E80 C. No. 3 pp. 378-387 March 1997, in detail.

As another SOI technique attracting attention, there is a techniquecalled Smart-Cut (registered trademark of SOITEC Co.). The Smart-Cutmethod is a technique developed by SOITEC Co. in France in 1996, and isa method of fabricating a bonded SOI substrate using hydrogenembrittlement. The particular technique of the Smart-Cut method isdisclosed in “Industrial Research Society (Kogyo Chosa Kai); ElectronicMaterial, August, pp. 83-87, 1977” in detail.

When the foregoing SOI substrate is fabricated, a single crystal siliconsubstrate having a main surface of a crystal face of a {100} plane(crystal orientation is <100> orientation) has been used in anytechnique. The reason is that the {100} plane has lowest interface statedensity (Qss) and is suitable for a field effect transistor that issensitive to interface characteristics.

However, with respect to the SOI substrate used for a TFT, since asingle crystal silicon thin film must be formed on an insulating layer,higher priority must be given to adhesion to the insulating layer thanthe interface state density. That is, even if the interface statedensity is low, it is meaningless if the single crystal silicon thinfilm peels off.

SUMMARY OF THE INVENTION

The present invention has been made in view of such problems, and anobject thereof is to provide a semiconductor device with highreliability by fabricating an SOI substrate suitable for a TFT and byforming TFTs on the substrate.

The structure of the present invention disclosed in the presentspecification is characterized by comprising the steps of:

forming a hydrogen-containing layer at a predetermined depth in a singlecrystal semiconductor substrate having a main surface of a {110} plane;

bonding the single crystal semiconductor substrate and a supportingsubstrate to each other;

splitting the single crystal semiconductor substrate by a first heattreatment along the hydrogen-containing layer;

carrying out a second heat treatment at a temperature of 900 to 1200°C.;

grinding a single crystal semiconductor layer remaining on thesupporting substrate and having a main surface of a {110} plane; and

forming a plurality of TFTs each having an active layer of the singlecrystal semiconductor layer.

Further, another structure of the present invention is characterized bycomprising the steps of:

forming a porous semiconductor layer by anodization of a single crystalsemiconductor substrate having a main surface of a {110} plane;

carrying out a heat treatment to the porous semiconductor layer in areducing atmosphere;

carrying out epitaxial growth of a single crystal semiconductor layerhaving a main surface of a {110} plane on the porous semiconductorlayer; bonding the single crystal semiconductor substrate and asupporting substrate to each other;

carrying out a heat treatment at a temperature of 900 to 1200° C.;

grinding the single crystal semiconductor substrate until the poroussemiconductor layer is exposed;

removing the porous semiconductor layer to expose the single crystalsemiconductor layer; and

forming a plurality of TFTs each having an active layer of the singlecrystal semiconductor layer on the supporting substrate.

Still further, another structure of the present invention ischaracterized by comprising the steps of:

forming an oxygen-containing layer at a predetermined depth in a singlecrystal semiconductor substrate having a main surface of a {110} plane;

changing the oxygen-containing layer into a buried insulating layer by aheat treatment; and

forming a plurality of TFTs each having an active layer of a singlecrystal semiconductor layer having a main surface of a {110} plane onthe buried insulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F are views showing fabricating steps of an SOI substrateof Embodiment 1.

FIGS. 2A to 2E are views showing fabricating steps of a TFT ofEmbodiments.

FIGS. 3A to 3F are views showing fabricating steps of an SOI substrateof Embodiment 2.

FIGS. 4A to 4C are views showing fabricating steps of an SOI substrateof Embodiment 3.

FIGS. 5A to 5C are views showing a structure of a semiconductor device(electro-optical device) of Embodiment 4.

FIG. 6 is view showing a structure of a semiconductor device(semiconductor circuit) of Embodiment 5.

FIGS. 7A to 7F are views showing structures of semiconductor devices(electronic equipments) of Embodiment 6.

FIGS. 8A to 8D are views showing structures of semiconductor devices(electronic equipments) of Embodiment 6.

FIGS. 9A and 9B are photographs showing a crystal structure of singlecrystal silicon of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The gist of the present invention is to use a single crystalsemiconductor substrate having a main surface of a {110} plane (crystalface is a {110} plane) as a forming material of a single crystalsemiconductor layer finally formed on a supporting substrate when an SOIsubstrate is fabricated by using an SOI technique such as SIMOX. ELTRAN,or Smart-Cut.

Incidentally, although the semiconductor mentioned here typicallyindicates silicon, the term also includes other semiconductors such assilicon germanium.

The reason why a single crystal semiconductor substrate having a mainsurface of a {110} plane is used as a forming material of a singlecrystal semiconductor layer will be described below. Incidentally, thisdescription will be made using single crystal silicon as an example.

As single crystal silicon, although that formed by an FZ method and thatformed by a CZ method exist, in the present invention, it is preferableto use single crystal silicon formed by the FZ method. In the CZ methodwhich is the main stream at present, oxygen of about 2×10¹⁸ atoms/cm³ iscontained for the purpose of relieving stress, so that there is a fearthat an electron or hole mobility is lowered. Particularly, in the casewhere a minute TFT is formed, this comes to appear remarkably.

However, in the case where single crystal silicon is used for the SOIsubstrate as in the present invention, since there are many cases wherethe thickness of a single crystal silicon layer required for an activelayer of a TFT is as very thin as 10 to 50 nm, it is not quite necessaryto take stress into consideration. Thus, even if the FZ method (oxygencontent is 1×10¹⁷ atoms/cm³ or less), which can form single crystalsilicon more inexpensively than the inexpensive CZ method, is used,satisfactory effects can be obtained.

In a general SOI substrate, a single crystal silicon layer is formed ona silicon oxide film. Thus, adhesion and conformity between the siliconoxide layer and the single crystal silicon layer become important. Fromsuch a viewpoint, in the SOI substrate, when the single crystal siliconlayer comes in contact with the silicon oxide layer, it is ideal thatthe contact of the single crystal silicon layer is realized with themost stable plane.

The plane which is in contact with the silicon oxide layer with moststably is a {110} plane. Because, in the case of the {110} plane, theplane is in contact with the silicon oxide layer through three siliconatoms. This state will be explained with reference to photographs shownin FIGS. 9A and 9B.

FIG. 9A is a photograph of a crystal structure model showing the statewhere two unit lattices of single crystal silicon are placed side byside. Here, a noticeable point is a portion indicated by an arrow in thedrawing. In the portion indicated by the arrow, three silicon atoms areplaced side by side. Any of the three silicon atoms is contained in aplane of the {110} plane.

That is, when a single crystal silicon layer having a crystal face ofthe {110} plane is formed on an insulating layer, it is understood thatthe number of silicon atoms coming in contact with the insulating layeris three.

FIG. 9B is a photograph showing the state of FIG. 9A seen in a differentangle. In FIG. 9B, although three silicon atoms exist in a portionindicated by an arrow, these are the same as the three silicon atomsindicated by the arrow in FIG. 9A.

Like this, it is understood that three silicon atoms are contained inthe {110} plane, and are adjacently arranged in a substantiallytriangular shape. That is, in such an arrangement state, the singlecrystal silicon layer is in contact with an insulating layer as an underlayer, and forms stable contact which is realized through “surface”.This indicates that the single crystal silicon layer is in contact withthe insulating layer as an under layer with very high adhesion.

On the other hand, in the case where the single crystal silicon comes incontact with the silicon oxide layer through another plane, for example,a {100} plane or a {111} plane, the number of silicon atoms coming incontact with the silicon oxide layer is at most two, and unstablecontact is formed in which the contact is realized through “line”.

Further, as a great merit of using the single crystal silicon layerhaving the main surface of the {110} plane, it is possible to mentionthat a silicon surface is very flat. In the case where the main surfaceis the {110} plane, a cleavage plane appears lamellary, and it ispossible to form a surface with very few asperities.

Like this, in the present invention, first priority is given to adhesionof a single crystal silicon layer to an under layer (silicon oxidelayer) in the SOI substrate, and the invention is characterized by usingthe single crystal silicon substrate having the crystal face of the{110} plane which has not been conventionally used. That is, theinvention is characterized in that the single crystal semiconductorsubstrate having the main surface (crystal face) of the {110} plane isused as a material, and the SOI technique such as SIMOX, ELTRAN, orSmart-Cut is fully used, so that the SOI substrate with high reliabilityis formed. Incidentally, an oriental flat of the single crystalsemiconductor substrate having the main surface of the {110} plane maybe made a {111} plane.

Then such an SOI substrate is used, and a plurality of TFTs each havingan active layer of a single crystal semiconductor thin film are formedon the same substrate, so that a semiconductor device having highreliability can be realized.

The present invention will next be described in detail with preferredembodiments described below.

Embodiment 1

In this embodiment, with reference to FIGS. 1A to 1F and 2A to 2E, adescription will be made on a case where when an SOI substrate isfabricated by a Smart-Cut method, a single crystal silicon substratehaving a main surface of a {110} plane is used, and a semiconductordevice is fabricated by using the SOI substrate.

First, a single crystal silicon substrate 101 as a forming material of asingle crystal silicon layer is prepared. Here, although a P-typesubstrate having a main surface of a crystal face of a {110} plane isused, an N-type substrate may be used. Of course, a single crystalsilicon germanium substrate may be used.

Next, a thermal oxidation treatment is carried out, so that a siliconoxide film 102 is formed on the main surface (corresponding to anelement forming surface). Although a film thickness may be suitablydetermined by a user, the thickness is made 10 to 500 nm (typically 20to 50 nm). This silicon oxide film 102 functions later as a part of aburied insulating layer of an SOI substrate (FIG. 1A). At this time, theadhesion between the single crystal silicon substrate 101 and thesilicon oxide film 102 becomes very high. Because, the silicon oxidefilm 102 is formed on the {110} plane in this invention, so that aninterface with very high conformity can be realized. Since thisinterface is an interface between an active layer and an under film in afinal TFT, it is very advantageous that the adhesion (conformity) ishigh.

The reason why the thickness of the silicon oxide film 102 can be madeas thin as 20 to 50 nm is that the crystal face of the single crystalsilicon substrate 101 has the {110} plane, so that the silicon oxidefilm having high adhesion can be formed even though it is thin.

Incidentally, the {110} plane has a problem that when an oxidationreaction proceeds, undulation (asperity) of the silicon surfacegradually becomes large. However, in the case where a thin silicon oxidefilm is provided as in this embodiment, since the amount of oxidation issmall, a problem of such undulation can be eliminated to the utmost.This is an advantage that is common to all embodiments disclosed in thepresent specification.

Thus, the single crystal silicon layer formed by using this inventionhas a very flat surface. For example, a distance between the top and topof the undulation is 10 times or less (preferably 20 times or less) aslong as a distance between adjacent atoms of the three atoms containedin the {110} plane. That is, it is about 5 nm or less (preferably 10 nmor less).

Next, hydrogen is added through the silicon oxide film 102 from the sideof the main surface of the single crystal silicon substrate 101. In thiscase, the hydrogen addition may be carried out as the form of hydrogenions using an ion implantation method. Of course, the addition step ofhydrogen may be carried out by other means. In this way, ahydrogen-containing layer 103 is formed. In this embodiment, a hydrogenion with a dosage of 1×10¹⁶ to 1×10¹⁷ atom/cm² is added (FIG. 1B).

Since the depth where the hydrogen-containing layer is formed determinesthe thickness of the single crystal silicon layer later, precise controlis required. In this embodiment, control of a hydrogen addition profilein the depth direction is made so that the single crystal silicon layerwith a thickness of 50 nm remains between the main surface of the singlecrystal silicon substrate 101 and the hydrogen-containing layer 103.

Since the {110} plane is a plane which has the lowest atomic density,even if hydrogen ions are added, a probability of collision with siliconatoms is lowest. That is, it is possible to suppress damage at the timeof ion addition to the minimum.

Next, the single crystal silicon substrate 101 and a supportingsubstrate are bonded to each other. In this embodiment, a siliconsubstrate 104 is used as the supporting substrate, and a silicon oxidefilm 105 for bonding is provided on its surface. As the siliconsubstrate 104, it is satisfactory if an inexpensive silicon substrateformed by the FZ method is prepared. Of course, it does not matter if apolycrystal silicon substrate is used. Besides, if only flatness can beassured, a highly refractory substrate such as a quartz substrate, aceramic substrate, or a crystallized glass substrate may be used (FIG.1C).

At this time, since a bonding interface is formed of highly hydrophilicsilicon oxide films, they are adhered to each other with hydrogen bondsby reaction of moisture contained in both the surfaces.

Next, a heat treatment (first heat treatment) at 400 to 600° C.(typically 500° C.) is carried out. By this heat treatment, in thehydrogen-containing layer 103, a volume change of a minute vacancyoccurs, and a broken surface is produced along the hydrogen-containinglayer 103. By this, the single crystal silicon substrate 101 is split,so that the silicon oxide film 102 and a single crystal silicon layer106 are made to remain on the supporting substrate (FIG. 1D).

Next, as a second heat treatment, a furnace annealing step is carriedout in a temperature range of 1050 to 1150° C. In this step, at thebonded interface, stress relaxation of Si—O—Si bonds occurs, so that thebonded interface becomes stable. That is, this becomes a step ofcompletely bonding the single crystal silicon layer 106 to thesupporting substrate. In this embodiment, this step is carried out at1100° C. for 2 hours.

The bonded interface is stabilized in this way, so that a buriedinsulating layer 107 is defined. In FIG. 1E, a dotted line in the buriedinsulating layer 107 indicates the bonded interface, and means thatadhesion of the interface has become strong.

Next, the surface of the single crystal silicon layer 106 is flattened.For flattening, a polishing step called CMP (Chemical MechanicalPolishing) or a furnace annealing treatment at high temperature (about900 to 1200° C.) in a reducing atmosphere may be carried out.

The final thickness of the single crystal silicon layer 106 may be made10 to 200 nm (preferably 20 to 100 nm).

Next, the single crystal silicon layer 106 is patterned to form anisland-like silicon layer 108 which becomes an active layer of a TFT. Inthis embodiment, although only one island-like silicon layer is shown, aplurality of layers are formed on the same substrate (FIG. 1F).

In the manner as described above, the island-like silicon layer 108having the main surface of the {110} plane is obtained. The presentinvention is characterized in that the island-like silicon layerobtained in this way is used as an active layer of a TFT, and aplurality of TFTs are formed on the same substrate.

Next, a method of forming a TFT will be described with reference toFIGS. 2A to 2E. First, steps up to the state of FIG. 1F are completed.In FIG. 2A, although a supporting substrate 201 is actually divided intothe silicon substrate 104 and the buried insulating layer 107 in FIG. 1,they are shown in an integrated state for simplicity. An island-likesilicon layer 202 of FIG. 2A corresponds to the island-like siliconlayer 108 of FIG. 1F.

Next, a thermal oxidation step is carried out so that a silicon oxidefilm 203 with a thickness of 10 nm is formed on the surface of theisland-like silicon layer 202. This silicon oxide film 203 functions asa gate insulating film. After the gate insulating film 203 is formed, apolysilicon film having conductivity is formed thereon, and a gatewiring line 204 is formed by patterning (FIG. 2A).

Incidentally, in this embodiment, although the polysilicon film havingN-type conductivity is used as the gate wiring line, the material is notlimited to this. Particularly, for the purpose of decreasing theresistance of the gate wiring line, it is also effective to use a metalmaterial such as tantalum, tantalum alloy, or a laminate film oftantalum and tantalum nitride. Moreover, for the purpose of obtainingthe gate wiring line with further low resistance, it is also effectiveto use copper or copper alloy.

After the state of FIG. 2A is obtained, an impurity to give N-typeconductivity or P-type conductivity is added to form an impurity region205. The impurity concentration at this time determines the impurityconcentration of an LDD region later. In this embodiment, althougharsenic with a concentration of 1×10¹⁸ atoms/cm³ is added, neither animpurity nor a concentration is not required to be limited to thisembodiment.

Next, a thin silicon oxide film 206 with a thickness of about 5 to 10 nmis formed on the surface of the gate wiring line. This may be formed byusing a thermal oxidation method or a plasma oxidation method. Theformation of the silicon oxide film 206 has an object to make itfunction as an etching stopper in a next side wall forming step.

After the silicon oxide film 206 as an etching stopper is formed, asilicon nitride film is formed and etch back is carried out, so that aside wall 207 is formed. In this way, the state of FIG. 2B is obtained.

In this embodiment, although the silicon nitride film is used as theside wall 207, a polysilicon film or an amorphous silicon film may beused. Of course, it is needless to say that if the material of the gatewiring line is changed, room for choice of a material which can be usedas the side wall is widened.

Next, an impurity having the same conductivity as the former step isadded again. The concentration of the impurity added at this time ismade higher than that at the former step. In this embodiment, arsenic isused as the impurity, and the concentration is made 1×10²¹ atoms/cm³.However, it is not necessary to make limitation to this. By the additionstep of the impurity, a source region 208, a drain region 209, an LDDregion 210, and a channel formation region 211 are defined (FIG. 2C).

In this way, after the respective impurity regions are formed,activation of the impurity is carried out by furnace annealing, laserannealing, lamp annealing, or the like.

Next, silicon oxide films formed on the surfaces of the gate wiring line204, the source region 208, and the drain region 209 are removed toexpose their surfaces. Then a cobalt film 212 with a thickness of about5 nm is formed and a thermal treatment step is carried out. By this heattreatment, a reaction of cobalt and silicon occurs, so that a silicidelayer (cobalt silicide layer) 213 is formed (FIG. 2D).

This technique is a well-known salicide technique. Thus, it does notmatter if titanium or tungsten is used instead of cobalt, and a heattreatment condition and the like may be referred to the well-knowntechnique. In this embodiment, the heat treatment step is carried out byusing lamp annealing.

After the silicide layer 213 is formed in this way, the cobalt film 212is removed. Thereafter, an interlayer insulating film 214 with athickness of 1 μm is formed. As the interlayer insulating film 214, asilicon oxide film, a silicon nitride film, a silicon nitride oxidefilm, or a resin film, such as polyamide, polyimide, acryl, etc., may beused. Alternatively, these insulating films may be laminated.

Next, contact holes are formed in the interlayer insulating film 214,and a source wiring line 215 and a drain wiring line 216 made of amaterial containing aluminum as its main ingredient are formed. Finally,the entire component is subjected to furnace annealing at 300° C. for 2hours in a hydrogen atmosphere, and hydrogenating is completed.

In this way, a TFT as shown in FIG. 2E is obtained. Incidentally, thestructure described in this embodiment is merely an example, and a TFTstructure to which the present invention can be applied is not limitedto this. Thus, the invention can be applied to a TFT of any well-knownstructure. Besides, the step condition of this embodiment is merely anexample, and a user may properly determine an optimum condition otherthan the essential portion of the invention.

Besides, in this embodiment, although the description has been made withthe N-channel TFT as an example, it is also easy to fabricate aP-channel TFT. Further, it is also possible to form a CMOS circuit byforming an N-channel TFT and a P-channel TFT on the same substrate andby complementarily combining them.

Further, in the structure of FIG. 2E, if a pixel electrode (not shown)electrically connected to the drain wiring line 216 is formed bywell-known means, it is also easy to form a pixel switching element ofan active matrix type display device.

That is, the invention is also a very effective technique as a method offabricating an electro-optical device typified by a liquid crystaldisplay device, an EL (electroluminescence) display device, an EC(electrochromic) display device, a photoelectric conversion device(optical sensor), and the like.

Embodiment 2

In this embodiment, a description will be made of an example in which anSOI substrate different from that of embodiment 1 is fabricated by usinga single crystal silicon substrate having a main surface of a {110}plane, and a semiconductor device is fabricated by using the SOIsubstrate using FIGS. 3A to 3F. Specifically, a case where a techniquecalled ELTRAN is used will be described.

First, a single crystal silicon substrate 301 having a main surface(crystal face) of a {110} plane is prepared. Next, the main surface issubjected to anodization to form a porous silicon layer 302. Theanodization step may be carried out in a mixed solution of hydrofluoricacid and ethanol. The porous silicon layer 302 is regarded as a singlecrystal silicon layer provided with columnar surface holes at a surfacedensity of about 10¹¹ holes/cm³, and succeeds to the crystal state(orientation, etc.) of the single crystal silicon substrate 301 as itis. Incidentally, since the ELTRAN method itself is well known, thedetailed description will be omitted here.

After the porous silicon layer 302 is formed, it is preferable to carryout a heat treatment step in a reducing atmosphere and within atemperature range of 900 to 1200° C. (preferably 1000 to 1150° C.). Inthis embodiment, a heat treatment at 1050° C. for 2 hours is carried outin a hydrogen atmosphere.

As the reducing atmosphere, although a hydrogen atmosphere, an ammoniaatmosphere, or an inert gas atmosphere containing hydrogen or ammonia(mixed atmosphere of hydrogen and nitrogen, or hydrogen and argon, etc.)is preferable, flattening of the surface of the crystalline silicon filmcan be made even in an inert gas atmosphere. However, when a reductionof a natural oxidation film is carried out by using a reducing action,many silicon atoms with high energy are generated and the flatteningeffect is resultantly increased. Thus, use of the reducing atmosphere ispreferable.

However, attention needs to be especially paid to the point that theconcentration of oxygen or an oxygen compound (for example, OH radical)contained in the atmosphere must be 10 ppm or less (preferably 1 ppm orless). Otherwise, the reducing reaction by hydrogen comes not to occur.

At this time, in the vicinity of the surface of the porous silicon layer302, the surface holes are filled up by movement of silicon atoms, sothat a very flat silicon surface can be obtained.

Next, a single crystal silicon layer 303 is epitaxially grown on theporous silicon layer 302. At this time, since the epitaxially grownsingle crystal silicon layer 303 reflects the crystal structure of thesingle crystal silicon substrate 301 as it is, its main surface becomesa {110} plane. The film thickness may be 10 to 200 nm (preferably 20 to100 nm) (FIG. 3A).

Next, the single crystal silicon layer 303 is oxidized to form a siliconoxide layer 304. As a forming method, it is possible to use thermaloxidation, plasma oxidation, laser oxidation, or the like. At this time,a single crystal silicon layer 305 remains (FIG. 3B).

Next, as a supporting substrate, a polycrystal silicon substrate 306provided with a silicon oxide layer on its surface is prepared. Ofcourse, a ceramic substrate, a quartz substrate, or a glass ceramicsubstrate each provided with an insulating film on its surface may beused.

After the preparation of the single crystal silicon substrate 301 andthe supporting substrate (polycrystal silicon substrate 306) iscompleted in this way, both of the substrates are bonded to each otherin such a manner that the respective main surfaces are opposite to eachother. In this case, the silicon oxide layer provided on each of thesubstrates functions as an adhesive (FIG. 3C).

After bonding is ended, a heat treatment step at a temperature of 1050to 1150° C. is next carried out, and the bonded interface made of boththe silicon oxide layers is stabilized. In this embodiment, this heattreatment step is carried out at 1100° C. for 2 hours. Incidentally, aportion indicated by a dotted line in FIG. 3C is the bonded interfaceafter adhering has been completely performed. The silicon oxide layersprovided on both the substrates are integrated by the heat treatment tobecome a buried insulating layer 307.

Next, the single crystal silicon substrate 301 is ground from the rearsurface side by mechanical polishing such as CMP, and the grinding stepis ended when the porous silicon layer 302 is exposed. In this way, thestate shown in FIG. 3D is obtained.

Next, the porous silicon layer 302 is subjected to wet etching and isselectively removed. As an etchant to be used, a mixed solution of ahydrofluoric acid solution and a hydrogen peroxide solution ispreferable. It is reported that a solution of a mixture of 49% HF and30% H₂O₂ at a ratio of 1:5 has a selecting ratio of a hundred thousandtimes or more between a single crystal silicon layer and a poroussilicon layer.

The state shown in FIG. 3E is obtained in this way. In this state, theburied insulating layer 307 is provided on the polycrystal siliconsubstrate 306, and a single crystal silicon layer 308 is formed thereon.

Although the SOI substrate is completed at this time, since minuteasperities exist on the surface of the single crystal silicon layer 308,it is desirable to carry out a heat treatment step in a hydrogenatmosphere to perform flattening. This flattening phenomenon occurs dueto speed-increasing surface diffusion of silicon atoms by reduction of anatural oxidation film.

At this time, since there is also an effect that boron contained in thesingle crystal silicon layer 308 (that contained in a P-type siliconsubstrate) is released into a vapor phase by hydrogen atoms, the heattreatment step is also effective in decrease of impurities.

Next, the obtained single crystal silicon layer 308 is patterned to forman island-like silicon layer 309. Although only one layer is shown inthe drawings, it is needless to say that a plurality of island-likesilicon layers may be formed.

Thereafter, a TFT can be fabricated in accordance with the same steps asthose described in embodiment 1 with reference to FIGS. 2A to 2E. TheTFT may be formed by other well-known means. In this embodiment, thedetailed description will be omitted.

Embodiment 3

In this embodiment, a description will be made on an example in which asingle crystal silicon substrate having a main surface of a {110} planeis used to fabricate an SOI substrate different from that of embodiment1 or embodiment 2, and a semiconductor device is fabricated by using thesubstrate using FIGS. 4A to 4C. Specifically, a case in which an SOIsubstrate called SIMOX is fabricated will be described.

In FIG. 4A, reference numeral 401 designates a single crystal siliconsubstrate. In this embodiment, an oxygen ion is first added to thesingle crystal silicon substrate 401 to form an oxygen-containing layer402 at a predetermined depth. The oxygen ion with a dosage of about1×10¹⁸ atoms/cm² may be added.

At this time, since the {110} plane has small atomic density, aprobability of collision of the oxygen ion and silicon atom becomeslower. That is, it is possible to suppress damage of the silicon surfacedue to the addition of oxygen to the minimum. Of course, if thesubstrate temperature is raised at 400 to 600° C. during the ionaddition, the damage can be further decreased.

Next, a heat treatment at a temperature of 800 to 1200° C. is carriedout, so that the oxygen-containing layer 402 is changed into a buriedinsulating layer 403. The width of the oxygen-containing layer 402 inthe depth direction is determined by a distribution of the oxygen ion atthe ion addition, and has a distribution with a gentle tail. However, bythis heat treatment, the interface between the single crystal siliconsubstrate 401 and the buried insulating layer 403 becomes very steep(FIG. 4B).

The thickness of this buried insulating layer 403 is 10 to 500 nm(typically 20 to 50 nm). The reason why the buried insulating layer asthin as 20 to 50 nm can be realized is that the interface between thesingle crystal silicon substrate 401 and the buried insulating layer 403is stably coupled, which is the very result of the fact that the singlecrystal silicon substrate having the main surface of the {110} plane isused as the material of the single crystal silicon layer.

When the buried insulating layer 403 is formed in this way, a singlecrystal silicon layer 404 remains on the buried insulating layer 403.That is, in this embodiment, since the single crystal silicon substratehaving the main surface of the {110} plane is used, the main surface(crystal face) of the single crystal silicon layer 404 obtained afterthe buried insulating layer is formed comes to have the {110} plane aswell. Incidentally, adjustment may be made so that the thickness of thesingle crystal silicon layer 404 becomes 10 to 200 nm (preferably 20 to100 nm).

After the single crystal silicon layer 404 is obtained in this way,patterning is carried out to obtain an island-like silicon layer 405. Aplurality of island-like silicon layers may be formed.

Thereafter, a plurality of TFTs may be completed in accordance with thesteps described in embodiment 1 using FIGS. 2A to 2E. The TFTs may beformed by other well-known means. In this embodiment, the detaileddescription will be omitted.

Embodiment 4

In this embodiment, an example of a reflection type liquid crystaldisplay device as a semiconductor device of the present invention willbe shown in FIGS. 5A to 5C. Since a fabricating method of a pixel TFT(pixel switching element) and a cell assembling step may be made byusing well-known means, the detailed description will be omitted.

In FIG. 5A, reference numeral 11 designates a substrate having aninsulating surface, 12 designates a pixel matrix circuit, 13 designatesa source driver circuit, 14 designates a gate driver circuit, 15designates an opposite substrate, 16 designates an FPC (Flexible PrintedCircuit), and 17 designates a signal processing circuit. As the signalprocessing circuit 17, a circuit performing processing for which an IChas been conventionally substituted, such as a D/A converter, aγ-correction circuit, or a signal dividing circuit, can be formed. Ofcourse, it is also possible to provide an IC chip on a glass substrateand to perform signal processing on the IC chip.

Further, although the description in this embodiment has been made onthe liquid crystal display device as an example, it is needless to saythat the present invention can be applied to an EL (electroluminescence)display device or an EC (electrochromic) display device as long as thedisplay device is an active matrix type display device. Here, an exampleof a circuit constituting the driver circuits 13 and 14 of FIG. 5A isshown in FIG. 5B. Since a TFT portion has been described in embodiment1, only necessary portions will be described here.

In FIG. 5B, reference numerals 501 and 502 designate N-channel TFTs, 503designates a P-channel TFT, and the TFTs 501 and 503 constitute a CMOScircuit. Reference numeral 504 designates an insulating layer made of alaminate film of a silicon nitride film/silicon oxide film/resin film,and a titanium wiring line 505 is provided thereon. The CMOS circuit andthe TFT 502 are electrically connected to each other. The titaniumwiring line is further covered with an insulating layer 506 made of aresin film. The two insulating films 504 and 506 also have a function asa flattening film.

A part of circuits constituting the pixel matrix circuit 12 of FIG. 5Ais shown in FIG. 5C. In FIG. 5C, reference numeral 507 designates apixel TFT made of a double gate structure N-channel TFT, and a drainwiring line 508 is formed to widely extend into a pixel region.

The insulating layer 504 is provided thereon, and the titanium wiringline 505 is provided thereon. At this time, a recess portion is formedin a part of the insulating layer 504, and only silicon nitride andsilicon oxide of the lowermost layer are made to remain. By this,auxiliary capacitance is formed between the drain wiring line 508 andthe titanium wiring line 505.

Besides, the titanium wiring line 505 provided in the pixel matrixcircuit has an electric field shielding effect between the source/drainwiring line and a subsequent pixel electrode. Further, it also functionsas a black mask at gaps between a plurality of pixel electrodesprovided.

Then, the insulating layer 506 is provided to cover the titanium wiringline 505, and a pixel electrode 509 made of a reflective conductive filmis formed thereon. Of course, it does not matter if contrivance to raisereflectivity may be made on the surface of the pixel electrode 509.

Actually, although an orientation film or a liquid crystal layer isprovided on the pixel electrode 509, the description will be omittedhere.

The reflection type liquid crystal display device having the structureas described above can be fabricated by using the present invention. Ofcourse, when combined with a well-known technique, it is also possibleto easily fabricate a transmission type liquid crystal display device.Further, when combined with a well-known technique, it is also possibleto easily fabricate an active matrix type EL display device.

Embodiment 5

The present invention can be applied to all conventional IC techniques.That is, the present invention can be applied to all semiconductorcircuits presently available on the market. For example, the presentinvention may be applied to a microprocessor such as a RISC processorintegrated on one chip or an ASIC processor, and may be applied tocircuits from a signal processing circuit such as a D/A converter to ahigh frequency circuit for a portable equipment (portable telephone,PHS, mobile computer).

FIG. 6 shows an example of a microprocessor. The microprocessor istypically constituted by a CPU core 21, a RAM 22, a clock controller 23,a cache memory 24, a cache controller 25, a serial interface 26, an I/Oport 27, and the like. Of course, the microprocessor shown in FIG. 6 isa simplified example, and various circuit designs are made for actualmicroprocessors according to their use.

However, in any microprocessor with any function, it is an IC(Integrated Circuit) 28 that functions as a central part. The IC 28 is afunctional circuit in which an integrated circuit formed on asemiconductor chip 29 is protected with ceramic or the like. AnN-channel TFT 30 and a P-channel TFT 31 having the structure of thisinvention constitute the integrated circuit formed on the semiconductorchip 29. Note that when a basic circuit is constituted by a CMOS circuitas a minimum unit, power consumption can be suppressed.

Besides, the microprocessor shown in this embodiment is mounted onvarious electronic equipments and functions as a central circuit. Astypical electronic equipments, a personal computer, a portableinformation terminal equipment, and other all household electricappliances can be enumerated. Besides, a computer for controlling avehicle (automobile, electric train, etc.) can also be enumerated.

Embodiment 6

A CMOS circuit and a pixel matrix circuit formed through carrying outthe present invention may be applied to various electro-optical devices(active matrix type liquid crystal display devices, active matrix typeEL display devices, active matrix type EC display devices). Namely, thepresent invention may be embodied in all the electronic equipments thatincorporate those electro-optical devices as display media.

As such an electronic equipment, a video camera, a digital camera, aprojector (rear-type projector or front-type projector), a head mountdisplay (goggle-type display), a navigation system for vehicles, apersonal computer, and a portable information terminal (a mobilecomputer, a cellular phone, or an electronic book) may be enumerated.Examples of those are shown in FIGS. 7A to 8D.

FIG. 7A shows a personal computer comprising a main body 2001, an imageinputting unit 2002, a display device 2003, and a key board 2004. Thepresent invention is applicable to the image inputting unit 2002, thedisplay device 2003, and other signal control circuits.

FIG. 7B shows a video camera comprising a main body 2101, a displaydevice 2102, a voice input unit 2103, an operation switch 2104, abattery 2105, and an image receiving unit 2106. The present invention isapplicable to the display device 2102, the voice input unit 2103, andother signal control circuits.

FIG. 7C shows a mobile computer comprising a main body 2201, a cameraunit 2202, an image receiving unit 2203, an operation switch 2204, and adisplay device 2205. The present invention is applicable to the displaydevice 2205 and other signal control circuits.

FIG. 7D shows a goggle-type display comprising a main body 2301, adisplay device 2302 and an arm portion 2303. The present invention isapplicable to the display device 2302 and other signal control circuits.

FIG. 7E shows a player that employs a recoding medium in which programsare recorded (hereinafter referred to as recording medium), andcomprises a main body 2401, a display device 2402, a speaker unit 2403,a recording medium 2404, and an operation switch 2405. Incidentally,this player uses as the recoding medium a DVD (digital versatile disc),a CD and the like to serve as a tool for enjoying music or movies, forplaying games and for connecting to the Internet. The present inventionis applicable to the display device 2402 and other signal controlcircuits.

FIG. 7F shows a digital camera comprising a main body 2501, a displaydevice 2502, an eye piece section 2503, an operation switch 2504, and animage receiving unit (not shown). The present invention is applicable tothe display device 2502 and other signal control circuits.

FIG. 8A shows a front-type projector comprising a display device 2601and a screen 2602. The present invention is applicable to the displaydevice and other signal control circuits.

FIG. 8B shows a rear-type projector comprising a main body 2701, adisplay device 2702, a mirror 2703, and a screen 2704. The presentinvention is applicable to the display device and other signal controlcircuits.

FIG. 8C is a diagram showing an example of the structure of the displaydevices 2601 and 2702 in FIGS. 8A and 8B. The display device 2601 or2702 comprises a light source optical system 2801, mirrors 2802 and 2805to 2807, dichroic mirrors 2803 and 2804, optical lenses 2808, 2809 and2811, liquid crystal display devices 2810, and a projection opticalsystem 2812. The projection optical system 2812 consists of an opticalsystem including a projection lens. This embodiment shows an example of“three plate type” using three liquid crystal display devices 2810, butnot particularly limited thereto. For instance, the invention may beapplied also to “single plate type”. Further, in the light pathindicated by an arrow in FIG. 8C, an optical system such as an opticallens, a film having a polarization function, a film for adjusting aphase difference, an IR film may be provided on discretion of a personwho carries out the invention.

FIG. 8D is a diagram showing an example of the structure of the lightsource optical system 2801 in FIG. 8C. In this embodiment, the lightsource optical system 2801 comprises light sources 2813 and 2814,synthetic prism 2815, collimator lenses 2816 and 2820, lens arrays 2817and 2818, polarizing converter element 2819.

The light source optical system shown in FIG. 8D employs two lightsources, but may employ three to four of light sources, or more. Ofcourse, it may employ one light source.

Further, on discretion of a person who carries out the invention, thelight source optical system may be provided with an optical system suchas an optical lens, a film having a polarization function, a film foradjusting the phase difference, and an IR film.

As described above, the scope of application of the present invention isvery wide, and the invention can be applied to electronic equipments ofany fields. The electronic equipment of this embodiment can be realizedeven if any combination of embodiments 1 to 5 is used.

1-12. (canceled)
 13. An information terminal comprising: a singlecrystalline semiconductor layer comprising silicon formed on aninsulating layer comprising silicon oxide over a supporting substrate,the single crystalline semiconductor layer having at least a channelformation region and source and drain regions; a gate insulating filmformed on the single crystalline semiconductor layer; a gate electrodeformed over the channel formation region with the gate insulating filminterposed therebetween wherein the gate electrode includes a metallayer which contacts the gate insulating film; etching stoppers formedon side surfaces of the gate electrode; side walls formed adjacent tothe side surfaces of the gate electrode with the etching stoppersinterposed therebetween; and an insulating film comprising siliconnitride formed over the single crystalline semiconductor layer and thegate electrode.
 14. The information terminal according to claim 13wherein the source and drain regions are in contact with the insulatinglayer.
 15. The information terminal according to claim 13 furthercomprising a first LDD region between the channel region and the drainregion and a second LDD region between the channel region and the sourceregion, wherein the first and second LDD regions are in contact with theinsulating layer.
 16. The information terminal according to claim 13wherein the single crystalline semiconductor layer is 20 to 100 nmthick.
 17. An information terminal comprising: a single crystallinesemiconductor layer comprising silicon formed on an insulating layerover a supporting substrate, the single crystalline semiconductor layerhaving at least a channel formation region and source and drain regions;a gate insulating film formed on the single crystalline semiconductorlayer; a gate electrode comprising poly silicon formed over the channelformation region with the gate insulating film interposed therebetween;etching stoppers formed on side surfaces of the gate electrode whereinthe etching stoppers comprise silicon oxide; side walls comprisingsilicon nitride formed adjacent to the side surfaces of the gateelectrode with the etching stoppers interposed therebetween; and aninsulating film comprising silicon nitride formed over the singlecrystalline semiconductor layer and the gate electrode, wherein an uppersurface of the gate electrode and at least a part of the source anddrain regions comprise a metal silicide.
 18. The information terminalaccording to claim 17 wherein the information terminal is a portableinformation terminal.
 19. The information terminal according to claim 17wherein the information terminal is a portable computer.
 20. Theinformation terminal according to claim 17 wherein the informationterminal is a cellular phone.
 21. The information terminal according toclaim 17 wherein the metal silicide is cobalt silicide.
 22. Theinformation terminal according to claim 17 wherein the singlecrystalline semiconductor layer is hydrogenated.
 23. The informationterminal according to claim 17 wherein the single crystallinesemiconductor layer is island-like.
 24. The information terminalaccording to claim 17 wherein the source and drain regions are incontact with the insulating layer.
 25. The information terminalaccording to claim 17 wherein the single crystalline semiconductor layeris 20 to 100 nm thick.
 26. An information terminal comprising: a singlecrystalline semiconductor layer comprising silicon formed on aninsulating layer over a supporting substrate, the single crystallinesemiconductor layer having at least a channel formation region andsource and drain regions; a gate insulating film formed on the singlecrystalline semiconductor layer; a gate electrode formed over thechannel formation region with the gate insulating film interposedtherebetween wherein the gate electrode includes a metal layer whichcontacts the gate insulating film; etching stoppers formed on sidesurfaces of the gate electrode; side walls formed adjacent to the sidesurfaces of the gate electrode with the etching stoppers interposedtherebetween; and an insulating film comprising silicon nitride oxideformed over the single crystalline semiconductor layer and the gateelectrode.
 27. An information terminal comprising: a single crystallinesemiconductor layer comprising silicon formed on an insulating layerover a supporting substrate, the single crystalline semiconductor layerhaving at least a channel formation region and source and drain regions;a gate insulating film formed on the single crystalline semiconductorlayer; a gate electrode comprising poly silicon formed over the channelformation region with the gate insulating film interposed therebetween;etching stoppers formed on side surfaces of the gate electrode whereinthe etching stoppers comprise silicon oxide; side walls comprisingsilicon nitride formed adjacent to the side surfaces of the gateelectrode with the etching stoppers interposed therebetween,respectively; and an insulating film comprising silicon nitride oxideformed over the single crystalline semiconductor layer and the gateelectrode, wherein an upper surface of the gate electrode and at least apart of the source and drain regions comprise a metal silicide.
 28. Aninformation terminal comprising: a single crystalline semiconductorlayer comprising silicon formed on an insulating surface, the singlecrystalline semiconductor layer having at least a channel formationregion and source and drain regions; a gate insulating film formed onthe single crystalline semiconductor layer; a gate electrode formed overthe channel formation region with the gate insulating film interposedtherebetween; etching stoppers formed on side surfaces of the gateelectrode; side walls formed adjacent to the side surfaces of the gateelectrode with the etching stoppers interposed therebetween; a firstinsulating film formed over the single crystalline semiconductor layerand the gate electrode; a first electrode formed on the first insulatingfilm and electrically connected to one of the source and drain regions;a second flattening insulating film formed over the first electrode andthe first insulating film; a second electrode formed on the secondflattening insulating film; and a third flattening insulating filmformed over the second electrode and the second flattening insulatingfilm.
 29. The information terminal according to claim 28 wherein thegate electrode includes a metal layer which contacts the gate insulatingfilm.
 30. The information terminal according to claim 29 wherein thegate electrode comprises tantalum or a tantalum nitride.
 31. Theinformation terminal according to claim 28 wherein the etching stopperscomprise silicon oxide and the side walls comprise silicon nitride. 32.The information terminal according to claim 28 wherein the firstinsulating film comprises silicon nitride.
 33. The information terminalaccording to claim 28 wherein the second and third flattening insulatingfilms comprise a resin.
 34. The information terminal according to claim28 wherein an upper surface of the gate electrode and at least a part ofthe source and drain regions comprise a metal silicide.
 35. Theinformation terminal according to claim 34 wherein the metal silicide iscobalt silicide.
 36. The information terminal according to claim 28wherein the information terminal is a portable computer.
 37. Theinformation terminal according to claim 28 wherein the informationterminal is a cellular phone.